The present invention is directed to hydrocarbon conversion processes by use of supported heteropoly acids as catalysts. In particular, the invention relates to isomerisation of C4-C24 paraffinic hydrocarbons.
Paraffin isomerisation is a widely used petrochemical process, which is applied mainly for increasing octane number of the C5/C6 alkane fraction in the products of crude oil processing. It is also applied, for example, in preparation of a feedstock for alkylation, e.g. butane isomerisation, in dewaxing of lube oils or in decreasing the boiling point or pour point of diesel fractions.
The invention is a process for conversion of hydrocarbon feed by contacting the feed with a catalyst containing heteropoly acid supported on a carrier at reaction conditions being effective in the conversion of the feed, wherein the carrier is selected from substantially inert inorganic amorphous or crystalline material, which retains characteristic structure of the heteropoly acid as evidenced by vibration frequencies around 985 and 1008 cmxe2x88x921, and which has a surface area larger than 15 m2/g excluding surface area in pores below 15 xc3x85 in diameter.
The term heteropoly acid as used herein shall mean an acid, wherein two or more inorganic oxyacids are condensed together. The central elements of these compounds can be selected from the group consisting of P, Si, B, Ge, As, Se, Ti and Zr. As coordinating elements of these acids, such ions as Mo, W, V, Mn, Co, Ni, Cu, Zn and Fe may be used. An example of such an acid is a compound of the general formula: H3AB12O40, where atom A is most preferably phosphorus and atom B can be most preferably W or Mo. Heteropoly acids being useful in the invention may further be various other heteropoly acids being effective catalysts in paraffin isomerisation and include H6As2Mo18O62, H6TeMo6O24 etc. Also, various salts of heteropoly acids can be used, e.g. salts of Na, Cs, Ba. The salt must contain some remaining protons and must be supported on an appropriate support.
A noble metal is one or a mixture of several metals from the Group VIII of the Periodic Table. An alloy containing at least one or more noble metals is furthermore useful in the isomerisation reaction. Among the metals, Pt and Pd are most preferable. The noble metals are supported on the surface of the catalyst in such a way that its dispersion rate is as high as possible. Various compounds can be used in preparation of the catalysts, for example, chlorides, nitrates, iodides, acetylacetonates, acetates, ammonia-containing complexes or chloroplatinic acid. The metal can be introduced using conventional methods, for example cation exchange, incipient wetness impregnation, solid state exchange or CVD, co-precipitation and co-impregnation. The content of the metal of Group VIII in the present invention is preferably of between 0.01% and 30% by weight, more preferably between 0.05% and 5%, and most preferably 0.1% to 1% based on the total weight of the catalyst.
Oxide materials useful as support material have low concentration of basic sites and do not react with the supported heteropoly acid, thus keeping the structure of the acid intact. Typical and preferable examples of such supports are high surface titania or zirconia compounds. Further useful materials comprise silica, alumina, ceria, lanthana, various mixed oxides, zeolites or other molecular sieves. Various non-oxide supports are additionally useful support materials, such as active carbon or other carbon-based carriers, carbides or nitrides, for example silicon carbide.
The heteropoly acid can be supported onto the carrier in any known method including impregnation with aqueous or non-aqueous solutions. The content of the heteropoly acid on the support may vary from 1% to 50%. The preferable content is between 5% and 30%, and more preferably between 10% and 25%.
Preliminary treatment of the catalyst precursor comprising a heteropoly acid supported onto an inert carrier includes calcination in an air, oxygen, nitrogen or inert gas flow at a temperature ranging from 200xc2x0 C. to 600xc2x0 C., preferably from 350xc2x0 C. to 500xc2x0 C. The final catalyst containing a noble metal is activated at a temperature ranging from 200xc2x0 C. to 600xc2x0 C., preferably from 350xc2x0 C. to 500xc2x0 C. in an air, nitrogen or inert gas flow. The catalyst may further be reduced in a hydrogen flow at a temperature from 100xc2x0 C. to 500xc2x0 C., or it can be used without preliminary reduction.
The purpose of isomerisation is to obtain branched isomers for various purposes, for example, to increase the octane number of light naphtha, decrease the viscosity of long-chain paraffins, decrease boiling point of higher paraffins or to prepare branched isomers as a feedstock for further processing. Paraffin isomerisation is typically carried out in a continuous flow or in a batch-wise reactor in the presence of a catalyst and preferably in the presence of hydrogen. The reaction temperature can vary in a range
100-450xc2x0 C. Temperatures out of this range can also be used, although they are less preferred. The most preferred temperatures for light naphtha isomerisation are in the range 100-250xc2x0 C. The reaction pressures are typically 1-40 bar. The preferable pressures are 10-20 bar. Isomerisation proceeds in the presence of hydrogen to increase selectivity of the isomerisation products, stability of the catalyst and to decrease gas formation. The typical hydrogen: feed molar ratio is 0.1 to 10, usually from 0.5 to 2. Hydrogen can be diluted with an inert gas like nitrogen or helium. The volume hourly space velocities are typically from 0.1 to 10 hxe2x88x921 and usually from 0.5 to 3 hxe2x88x921.
Linear n-paraffin feed such as n-butane, n-pentane, n-hexane, n-heptane, n-octane and higher paraffins (C9-C16) or a mixture thereof are usual substrates in the isomerisation process. The feed can also contain other hydrocarbons, such as aromatic or naphthenic hydrocarbons, which do not interfere with the isomerisation reaction.
In order to illustrate further the invention and the advantages thereof, the following specific examples are given.